It is known to dye human keratinous fibres and in particular hair with dyeing compositions containing oxidation dyes. Oxidation dyes comprise oxidation dye precursors and couplers.
Oxidation dye precursors, generally called oxidation bases, are colorless or weakly colored compounds which, combined with oxidizing products, can give rise to colored and dye compounds by a process of oxidative condensation. They are in particular ortho- or para-phenylene diamines, ortho- or para-aminophenols, or heterocyclic bases.
The shades obtained with these oxidation bases may be modified by combining the bases with couplers or color modifiers, the couplers being chosen in particular from aromatic meta-diamines, meta-aminophenols, meta-diphenols and certain heterocyclic compounds.
The variety of molecules used in oxidation bases and couplers allows a rich palette of colors to be obtained.
The oxidation dyeing method comprises applying to the fibres oxidation bases or a mixture of oxidation bases and couplers with an oxidizing agent, most often hydrogen peroxide, allowing them to act, and then rinsing the fibres. The application, which is generally carried out at a basic pH, makes it possible to obtain dyeing and simultaneously lightening of the fibre, which results in practice in the possibility of obtaining a final coloration which is lighter than the original color. In addition, the lightening of the fibre has the advantageous effect of generating a uniform color in the case of grey hair, and in the case of naturally pigmented hair, of making the color stand out, that is to say of making it more visible.
The lightening of hair is evaluated by the tone height which characterizes the degree or level of lightening. The notion of “tone” is based on the classification of natural shades, a tone separating each shade from the one immediately following it or preceding it. This definition and the classification of natural shades is well known to hair styling professionals and is published in the book “Sciences des traitements capillaires” [Science of hair treatment] by Charles ZVIAK, 1988, Ed. Masson, pp. 215 and 278.
The tone heights range from 1 (black) to 10 (light blonde), one unit corresponding to one tone; the higher the figure, the lighter the shade.
The lightening oxidation dyeing technology which has to make it possible to obtain sufficient lightening of the fibre and a covering of hair which is 100% white has up until now involved using either aqueous ammonia, or monoethanolamine, or a mixture of monoethanolamine and aqueous ammonia, as alkalinizing agent.
However, as everyone knows, aqueous ammonia has the major disadvantage of releasing an unpleasant odor during application of the dye.
Monoethanolamine, if used in high concentrations, sometimes causes irritations of the scalp in the form of pricklings.
Now, after major research studies carried out on the subject, the inventors have discovered that it is possible to reduce the unpleasant odor and the risks of scalp irritation of the dyes while obtaining the desired lightening level and intense colorations in varied shades, using, as alkalinizing agent, a mixture of at least one alkali metal or alkaline-earth metal or ammonium metasilicate and at least one alkanolamine.